3. To obtain the desired radiation pattern, several dipole elements may be mounted on the web. The most
common arrangement though, is a single dipole with a reflecting element. The reflecting element can be either
another dipole or some type of reflecting material shaped according to the beam pattern desired.
LEARNING EVENT 9: TAPERED HORN
1. The gain of a waveguide radiator may be increased by enlarging the aperture. This is done by attaching a
flare or horn to the waveguide as shown in Figure 2-8. The waveguide termination is commonly known as a
tapered horn antenna. The tapered horn antenna is designed to transform a transverse wave at the end of the
waveguide to a similar transverse wave at the end of the tapered horn without causing attenuation. The throat of
the tapered horn (the junction between the tapered horn and the waveguide) serves as a filter device and allows
only a single mode to be propagated freely to the aperture. The tapered horn will not support propagation of a
particular mode unless the transverse dimensions of the tapered horn are greater than the dimensions of the
waveguide.
Figure 2-8. Tapered horn antenna.
2. The dimensions of the open end of the tapered horn are chosen to obtain the desired radiation pattern and to
prevent spherical distortion of the propagated wave. The taper of the horn serves to match the impedance of the
waveguide to the impedance of space. At one end, the impedance of the tapered section matches that of space;
at the other end, it matches the impedance of the waveguide.
LEARNING EVENT 10: REFLECTOR FEED SYSTEM
1. Since microwave frequencies have essentially the same behavior as light waves, they can be focused into
beams. One of the reflectors used to focus the energy into a narrow beam is a paraboloid. The focal point and
the contour of the reflector determine the size of the reflected beam. The paraboloid reflector is often called a
parabolic reflector.
2-9
SS0031
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